1. Field of the Invention
The present invention relates to a glass composition used in an optical device, and more particularly, to a glass composition for an optical fiber used in light amplification. The invention also relates to an apparatus for optical communication using the glass composition.
2. Description of the Related Art
An optical device material used in a light source such as a single wavelength laser oscillator used for optical communications, a superluminescent source of light, and an optical amplifier has been developed. However, an optical fiber compatible with amplifying a 1.3 .mu.m wavelength signal, which is the zero dispersion waveband of silica glass, has not been successfully developed.
In making one type of an optical fiber used for amplification of a 1.3 .mu.m wavelength signal, two rare earth elements are used as the active material. Neodymium (Nd) or praseodymium (Pr) is doped on a host glass to make a glass composition. By way of definition, glass which is not doped with an active material will be referred to as host glass. A glass composition represents host glass doped with an active material. In this case, the rare earth element is doped in an ion state, such as Nd.sup.3+ or Pr.sup.3+ ion, on a host glass such as silica glass.
However, when the Pr.sup.3+ ion is excited and emits light, the energy of the Pr.sup.3+ ion in a glass composition can be relaxed by a lattice vibration of the host glass, for example, in host glass made of silica. Since the optical amplification efficiency decreases as the probability of generating the relaxation becomes higher, a material having a low lattice vibration energy is required to decrease the probability of relaxation.
A three-component system glass composed of sulfur-rich Ge, Ga, and S can be used as a host glass having the low lattice vibration energy, as disclosed in U.S. Pat. No. 5,379,149, to Snitzer et al., entitled Glass Composition Having Low Energy Phonon Spectra And Light Sources Fabricated Therefrom. The patent discusses a host glass having a composition in which excess S is added in a ratio higher than S ratio on a composition line which connects GeS.sub.2 and Ga.sub.2 S.sub.3 in a ternary system phase diagram of germanium (Ge), gallium (Ga) and sulfur (S). This is referred to as a sulfur-rich glass. However, the center of the optical gain distribution of the glass composition made using this host glass and Pr.sup.3+ as active material is located at a 1,330 nm bandwidth. Therefore, a low optical gain is obtained in a 1,310 nm bandwidth which is the required optical communications bandwidth. Accordingly, the optical amplification efficiency is remarkably reduced.
The central wavelength of the optical gain distribution is determined by the difference of energy between a .sup.1 G.sub.4 state which is an excited state of the Pr.sup.3+ ion and a .sup.3 H.sub.5 state which is a metastable state. Such a difference of energy is smaller in a sulfide host glass than in an oxide host glass. Therefore, in the sulfide host glass, the central wavelength of the optical gain distribution is nearer to 1,310 nm which is the desired optical communications waveband.
Based on our observation of the art, then, we have found that what is needed is a glass composition which has a low lattice vibration energy and which has a high optical gain at the 1,310 nm bandwidth. Such a glass composition would have excellent optical amplification properties.